1. Field of the Invention
The present invention relates to an insulating film-forming agent for a non-oriented electromagnetic steel sheet having a high space factor, which exhibits an excellent adhesion of the insulating film and an excellent blanking quality, and which is excellent in slip characteristics after stress relief annealing, adhesion of the insulating film and corrosion resistance, and a method for producing a non-oriented electromagnetic steel sheet by coating the steel sheet substrate with the insulating film-forming agent.
2. Description of the Prior Art
When a laminated iron core for a motor, a transformer, etc. is produced with a non-oriented electromagnetic steel sheet, unit iron cores are usually prepared from the non-oriented electromagnetic steel sheet by shearing or blanking the sheet, stacked, and the stacked unit iron cores are fixed by bolting, caulking, welding, bonding, or the like procedure. The laminated iron core thus obtained is subjected to winding coil incorporation, or the like treatment, and finally a motor or transformer is assembled.
An insulating film is generally formed on a non-oriented electromagnetic steel sheet. Since the various properties of the steel sheet such as weldability, blanking quality and corrosion resistance greatly depend on the insulating film properties, it is important to impart not only insulating properties but also excellent film properties to the insulating film.
An inorganic, an organic, and an inorganic-organic mixture type insulating film have heretofore been known as the insulating film of a non-oriented electromagnetic steel sheet. However, the steel sheet having an inorganic insulating film exhibits a poor blanking quality compared with that having an organic insulating film or inorganic-organic mixture type insulating film. The steel sheet having an organic insulating film exhibits a poor adhesion of the film after stress relief annealing and a poor corrosion resistance compared with the steel sheet having an inorganic insulating film or inorganic-organic mixture type insulating film. Accordingly, the inorganic insulating film and the inorganic-organic mixture type insulating film cannot be used.
The inorganic-organic mixture type insulating films have been intensively investigated to solve the problems of the inorganic and the organic insulating films. Japanese Examined Patent (Kokoku) Publication No. 50-15013 proposes a method for forming an insulating film on a non-oriented electromagnetic steel sheet so that the resultant steel sheet exhibits film properties such as a high space factor, an excellent adhesion of the film and an excellent blanking quality, which method comprises forming an insulating film with a treatment solution containing a dichromate and an organic resin emulsion of a resin such as a vinyl acetate resin, a butadiene-styrene copolymer or acrylic resin as the principal components.
However, a Cr compound is used as a film component in the inorganic-organic mixture type insulating film of a conventional non-oriented electromagnetic steel sheet as observed in the use of a dichromate. Consequently, in the present situation where the production process of non-oriented electromagnetic steel sheets or customers of such non-oriented electromagnetic steel sheets are confronted with severe environmental problems, development of a technology for treating an insulating film containing no chromium compound is desired.
Japanese Unexamined Patent (Kokai) Publication No. 6-330338, therefore, discloses the following treatment method: a treatment method comprising mixing a phosphoric acid salt having a specific composition and an organic resin emulsion having a specific particle size in a specific proportion, coating a steel sheet with the resultant treatment solution, and baking finishing the steel sheet. The method uses a treatment solution containing no chromium compound, and the steel sheet obtained by the method shows film properties comparable to those of a conventional insulating film containing a chromium compound, and maintains excellent slip characteristics after stress relief annealing.
However, the technology disclosed in Japanese Unexamined Patent (Kokai) Publication No. 6-330338 has been found to have the following problem. The organic resin emulsion often agglomerates in the treatment solution or during the coating and baking step. As a result, portions containing no organic resin mingle with portions containing the organic resin in the insulating film, resulting in lowering the blanking quality of the steel sheet.
When an insulating film is to be formed on a non-oriented electromagnetic steel sheet, the steel sheet is usually coated with an insulating film treatment solution and baking finished subsequently to continuous annealing. It is, therefore, industrially important that the insulating film excellent in film properties can be stably formed over a long period of time without agglomerating the organic resin.
Furthermore, since many motors and transformers have complicated shapes, blanking conditions such as a clearance partially deviates from the optimum value sometimes. It has been found that the blanking quality lowers in such a case when the technology disclosed in Japanese Unexamined Patent (Kokai) Publication No. 6-330338 is employed.